Prepare a grill using the snake method or for indirect heat. If using a charcoal grill, stack the unlit charcoals around the perimeter of the grill, about halfway, then light the first few at one end so the snake begins to burn slowly. (This results in a controlled 325 to 350 degrees F heat.) If using a gas grill, heat one side only to medium-high heat.
Toss the potatoes with 2 tablespoons of the oil in a large bowl. Season well with salt and pepper.
Lay out 2 sheets of aluminum foil in a cross and place the potatoes in the middle. Gather up the sides to create a foil packet, add 2 tablespoons of water and seal the foil packet.
Place the foil packet over direct heat and steam until the potatoes are cooked through, about 30 minutes. Open the foil packet and continue to cook until the potatoes take on some smoke and dry out, about 5 minutes.
Meanwhile, whisk together the peanuts, remaining 1/4 cup oil, vinegar, cilantro, mustard, celery seeds and sugar in a large bowl. Season with salt and pepper.
Remove the potatoes from the grill, add to the bowl with the vinaigrette and toss to coat.
(Alternatively, you can bake the potatoes in a 375 degrees F oven for 30 minutes.)